The present invention relates generally to a capacitively powered radio frequency identification device for use in a wireless data communication system.
Remotely powered electronic devices and related systems are known. For example, U.S. Pat. No. 5,099,277 issued to Geiszler et al. titled xe2x80x9cProximity Detecting Apparatusxe2x80x9d, discloses a remotely powered device which uses electromagnetic coupling to derive power from a remote power source and then uses both electromagnetic and capacitive coupling to transmit stored data to a receiver often collocated with the remote power source. Such remotely powered communication systems are commonly known in the field of radio frequency identification (xe2x80x9cRFIDxe2x80x9d). In such systems, the remote power source is commonly known as a xe2x80x9creader, or an exciter,xe2x80x9d while the remotely powered portable device is commonly known as a xe2x80x9cRFID tagxe2x80x9d.
Earlier RFID tags and systems primarily used electromagnetic coupling to power the RFID tag and couple the RFID tag with an exciter system and its associated receiver. The exciter generates an electromagnetic excitation signal used to power the RFID tag and cause the device to transmit a signal including stored information. The receiver then receives the signal produced by the RFID tag to demodulate and recover the data.
Known electromagnetic coupling mechanisms include an oscillator as part of the exciter and a coil antenna on both the exciter and the RFID tag. The RFID tag typically includes an electronic circuit, such as an integrated circuit and memory. By way of example, in an earlier system, excitation circuitry is connected to a coil antenna, which radiates excitation signals that are picked up by a coil antenna mounted on a tag that contains the electronic circuit. The excitation signals energize the circuit, when then provides an information-carrying signal that is transmitted to the receiver using electromagnetic or capacitive coupling.
One problem with the use of electromagnetic coupling between a RFID tag and either an exciter or a receiver has been the complexity involved in the manufacture of tags that employ a coil antenna. The spiral layout of a typical coil antenna makes the tag more difficult to produce and increases tag cost and size. The coil antennas require tight tolerances for efficient performance. Additionally, typical coil antennas have undesirable thermal compression characteristics that affect, in particular, the ability to create a flat tag or remote device that encompasses the coil.
RFID tags and associated systems have numerous uses. By way of example, RFID tags are frequently used for personal identification in automated gate sentry applications protecting secured buildings or areas. These tags often take the form of access control cards. Information stored on the RFID tag identifies the person seeking access to the secured building or area. Older automated gate sentry applications require the person accessing the building to insert or swipe their identification tag into or through a reader for the system to read the information from the identification tag. Newer RFID tag systems allow the tag to be read at a distance, thereby eliminating the need to insert or swipe an identification tag into or through a reader. Most typically, the user simply holds or places the tag near or in proximity to the reader or base station, which is coupled to a security system securing the building or area. The base station transmits an excitation signal to the tag that powers circuitry contained on the tag. The circuitry, in response to the excitation signal, communicates stored information from the tag to the base station, which receives and decodes the information. The information read is used by the security system to determine if access is appropriate. Also, RFID tags may be written remotely by an excitation signal appropriately modulated in a predetermined manner.
In addition to typical applications for access control of persons, RFID tags may be useful in electronic animal identification, baggage tracking, parcel tracking, inventory management applications, asset identification and tracking, personal computer access and security, and other applications involving identification of things. These applications involve transmitting stored information from a tag to an exciter/reader when the tag is brought within the excitation field of the exciter/reader. Also, these applications may involve writing information to a tag. RFID tags for these applications may need to be durable for long-term use or disposable, for temporary use.
In applications for identification of persons and things, bar codes are almost universally employed. Generation of the bar code is very inexpensive. However, one problem associated with bar codes and bar code readers is that the bar codes must be precisely aligned with the bar code reader in order to be read. Another problem with bar codes is that the bar codes may become unreadable as a result of damage, for example, exposure to moisture, or wear and tear from use. RFID tags address some of the shortcomings of bar codes.
In addition to the need to transmit stored information via radio frequency transmission, it is often desirable for an RFID tag to be very thin, very flat, flexible, semi-flexible, or rigid and to nevertheless be compatible with printing technologies, including but not limited to die sublimation printing, ink jet printing, and flexographic printing and the like. Prior RFID tags incorporating coils are limited in their ability to be flat and thin and low in cost. This has greatly limited their application in the market place, particularly in areas where the cost of the tag must be competitive with a barcode.
Thus, there is a need for a thin, flat, inexpensive RFID device.